Litter decomposition is a key process of nutrient and carbon cycling in terrestrial ecosystems. The decomposition process will likely be altered under ongoing climate change, both through direct effects on decomposer activity, and through indirect effects caused by changes in litter quality.

We studied how hydrological change indirectly affects decomposition via plant functional community restructuring caused by changes in plant species´ relative abundances (community weighted mean traits (CWM) and functional diversity). We further assessed how those indirect litter quality effects compare to direct effects. We set-up a mesocosm experiment, in which sown grassland communities and natural turf pieces were subjected to different hydrological conditions (dryness, waterlogging) for two growing seasons. Species-level mean traits were obtained from trait data bases and combined with species´ relative abundances to assess functional community restructuring. We studied decomposition of mixed litter from these communities in a common “litterbed”. These indirect effects were compared to effects of different hydrological conditions on soil respiration and on decomposition of standard litter (direct effects). Dryness reduced biomass production in sown communities and natural turf pieces, while waterlogging only reduced biomass in sown communities. Hydrological stress caused profound shifts in species´ abundances and consequently in plant functional community composition. Hydrologically stressed communities had higher CMW leaf dry matter content, lower CMW leaf nitrogen content and lower functional diversity. Lower CWM leaf N content and functional diversity were strongly related to slower decomposition. These indirect effects paralleled direct effects, but were larger and longer-lasting. Species mean traits from trait data bases had therefore considerable predictive power for decomposition.

Our results show that stressful soil moisture conditions, that are likely to occur more frequently in the future, quickly shifts species´ abundances. The resulting functional community restructuring will decelerate decomposition under hydrological stress.

Most data were collected in an experimental mesocosm approach. Plant material of mesocosms was harvested, weighed and litter material was taken to study decomposition using the litter-bag and litterbed approach. Litter weight loss was quantified and calculated (%). Further, we used plant trait data bases to calculate community weighted mean traits and functional diversity, based on trait values and relative abunances of species within plant communities. Soil respiration, the decomposition of standard material and consumption of baits in lamina-bait sticks were assessed in pots within the communities, that contained bare soil only. The latter data are to be found mainly in the appendix.

Missing values are indicated with NA.

Abbreviations:

pot= pot number of respective turf piece or sown community

cov2018=percent cover of species in turf pieces in May 2018 [%]

water= water availability levels

L= water logged

W= wet

M= mesic

dry= dry

PL= pulsed flooding

weight [g]= species-specific biomass within sown communities

rel. abundance [%]= relative abundance of species of sown communities in %, based on biomass data

species=species within sown communities

Am=Achillea millefolium

Ap=Alopecurus pratensis

Ae=Arrhenatherum elatius

Bh=Bromus hordeaceus

Bp=Bellis perennis

Cb=Crepis biennis

Cj=Centaure jacea

Dg=Dactylis glomerata

Fr=Festuca rubra

Ga=Gallium mollugo agg.

Pl=Plantago lanceolata

Pp=Poa pratensis

Td=Trifolium dubium

Tp=Triflium pratense

Vc=Vica cracca

FDis=Functional dispersion, measure for functional diversity

RaoQ=Rao´s quadratic entropy, measure for functional diversity

SLA=specific leaf area (LEDA data base) [mm2mg-1]

LDMC=leaf dry matter content (LEDA data base) [mg/g]

N=leaf nitrogen content (TRY data base) [%]

meadow origin= different sites, from which turf pieces were taken in November 2015

bait1-16=baits of lamina bait sticks froms upper (1) to lower (16), 1=eaten, 0=not eaten

eaten=sum of eaten baits per stick

not=non-eaten baits per stick

pool=pool in which bare soil pots for soil respiration and lamina-bait testes were placed

initial weight [g]=dry weight of litter that was placed in litter bags

remaining weight [g]=dry weight of plant material that remained in litter bags after the decomposition period

C=carbon in leaves, measured with elemental analyser [%]

N=leaf nitrogen content, measured with elemental analyser [%9

CNratio=ratio between C and N in leaves, measured with elemental analyser

system=plant system in pots:

turf pieces=turf pieces dug out from natural meadows prior to experiment

sown communities=communities sown with 15 different species and kept free from non-target species

monocultures=monocultures consisting of one species, sown in the same density as sown communities

single plants= pots with only one single plant in the middle